This invention relates to a metallic article including an integral end or tip portion subject to damage such as cracking. More particularly, it relates to a metallic article, for example a gas turbine engine blading member made of a metallic alloy, including a tip subject to tip separation or cracking during operation and propagation of such damage from the tip into underlying material.
Power generation apparatus such as turbine engines include blading members, for example blades and vanes, having a free end portion or tip disposed in juxtaposition with another component in a relatively moving or rotating relationship. Examples of such members include a rotating compressor blade and a rotating turbine blade, having an airfoil with an airfoil tip disposed opposite a stationary shroud or seal across a relatively narrow gap. Such a gap is designed to limit leakage of a working fluid, such as air and/or products of combustion, through the gap.
As is well known and widely described in connection with the turbine engine art, such a blading member can operate at and experiences cycles including relatively high rotational speeds, sometimes cycling to high temperatures. As a result, in addition to thermal expansion and contraction of the member, local and high tensile and vibratory stresses have been generated in the tip portion of blading members. Such stresses have developed to an extent that can result in the generation of separations or cracks starting in the blade tip and propagating into the adjacent, integral body of the member. Rubbing between such relatively moving members can enhance generation of separations and cracks. Examples of such conditions have been described in the art, for example in such U.S. Pat. No. as 5,620,307xe2x80x94Mannava et al (patented Apr. 15, 1997) and U.S. Pat. No. 5,826,453xe2x80x94Prevey, III (patented Oct. 27, 1998).
The Mannava et al. and the Prevey, III patents describe methods and apparatus for providing a compressive residual stress in a surface region, area or layer of the article, extending into the article from a treated surface. Mannava et al provide such region of compressive residual stress through use of a Laser Shock Peening method, extending stress into an airfoil from a laser shock peened surface. Prevey, III uses a surface burnishing operation in the form of a Low Plasticity Burnishing method. This induces compressive stress in a surface layer on the surface of members, for example to a depth of less than about 0.05xe2x80x3 as shown by the data in the drawings, while limiting cold working to less than about 3.5%, for the reasons described by Prevey, III.
The present invention, in one form, provides an article made of a metallic material and comprising a body and a body end portion integral with the body. The end portion comprises an integral band of the metallic material substantially through the entire end portion, the band being under a compressive stress greater than the body. One example of such an article is a blading member made of a metal alloy and comprising an airfoil having an integral airfoil tip comprising the band with the compressive stress. The band extends into and toward the airfoil to a depth selected from operational experience to be sufficient to resist operational damage.
One embodiment of the invention is a method of providing such band under compression by selecting the depth of the band into the end portion or airfoil tip, and performing roller deformation on the selected band until compressive stress is provided substantially through the entire band.